Reduction of nitrogen oxides from products of hydrocarbon combustion with air

ABSTRACT

Fuel is burned in a primary combustion zone so that a substantial quantity of unburned hydrocarbons, such as carbon monoxide (CO) is produced along with some nitrogen oxides (NOx) and whereby essentially no oxygen remains at the completion of the combustion process. The gaseous combustion products are conducted through a gas dispersion matrix or bed in which the unburned hydrocarbons and NOx react to produce carbon dioxide (CO2) and nitrogen (N2). Air is then injected into the gases in a secondary combustion zone to oxidize the residual unburned hydrocarbons to CO2 in which case the exhaust gases are substantially free of air polluting CO and NOx.

United States Patent 1191 La Haye et a1. Aug. 27, 1974 [54] REDUCTION OFNITROGEN OXIDES FROM 3,254,695 6/1966 Brodlin 431/328 Y RB 3,291,18212/1966 Dow et a1. 431/10 X S 0N 3,407,024 10/1968 Hirschberg et al.431/328 3,421,826 1/1969 Tope et al 431/328 Inventors: Paul G- I43 Haye,CaPe Ehzabeth, P Maine; Glenn D. Craig, Menomonee n Joseph L Turecek,952,673 3/ 1964 Great Bntam 431/10 Sh 0d, b th f W- Orewo o o ls PrimaryExaminerEdward G. Favors Asslgnw q -4 Mllwaukee, Wis Attorney, Agent, orFirm-Fred Wiviott [22] Filed: Jan. 6, 1972 ABSTRACT [21] Appl' 215762Fuel is burned in a primary combustion zone so that a Related US.Application Data substantial quantity of unburned hydrocarbons, such[63] Continuation-impart of S61. N0. 198,767, Nov. 15, as carbon monmhde18 Produced along Yvlth 1971, some n1trogen ox1des (NOx) and wherebyessentlally no oxygen remains at the completion of the combus- 521 US.Cl....-. 431/10, 431/326, 431/351 tion Process The gaseous combustionproducts are [51] Int. Cl. F231 9/00 conducted ough a gas dispersionmatrix or bed in 58 Field 6: Search 431/10, 2, 7, 326, 328, which theunburned hydrocarbons and NOX react to 431 /351; 23/2 produce carbondioxide (CO and nitrogen (N Air is then injected into the gases in asecondary combus- [56] Ref r n Cit d tion zone to oxidize the residualunburned hydrocar- UNITED STATES PATENTS bons t0 CO in which case theexhaust gases are sub- 1,846,978 2/1932 Parker et a1. 431/10 x stamlauyfree of ponmmgco and 2,895,297 7/1959 -Gardiner 431/10 X 20 Claim, 4Drawing Figures PATENi'Enwszmu smurf-'3' FIG. I

Pmimiumszmn SIEETZOFS NOE PAlENTEnmcznsu sumsnr F'IG.4

REDUCTION OF NITROGEN OXIDES FROM PRODUCTS OF HYDROCARBON COMBUSTIONWITH AIR CROSS REFERENCE TO RELATED APPLICATION This application is acontinuation-in-part of application Ser. No. 198,767 filed Nov. 15,1971.

BACKGROUND OF THE INVENTION This invention relates to reducing airpolluting agents such as nitrogen oxides and unburned hydrocarbons inthe exhaust gases of carbonaceous and hydrocarbon fuel burners.

SUMMARY OF THE INVENTION It is a primary object of this invention toreduce air pollution by reducing the NO, which is emitted to theatmosphere in the exhaust gases from carbonaceous and hydrocarbon fuelburners and to minimize the emission of carbon monoxide and otherunburned hydrocarbons as well.

A further object is to provide a gas purifying device in which NO isreacted with unburned hydrocarbons to produce harmless N and CO and inwhich the residual hydrocarbons are completely oxidized or burned bymeans of second stage combustion to thereby produce harmless CO whichmay be exhausted to the atmosphere along with the N A still furtherobject is to provide a combustion gas purifying device which does notadversely affect the efficiency of the combustion process.

In general terms, the present invention involves dividing a combustionchamber intotwo stages. In the first stage hydrocarbon fuel is burnedwith insufficient air to completely oxidize all of the carbon in thefuel in which case a substantial amount of unburned hydrocarbon isintentionally produced in the gaseous combustion products. Some noxiousnitrogen oxides are also produced but in relatively low amounts sinceproduction of these oxides is inhibited by the presence of unburnedhydrocarbons, such as CO. The stream of gaseous combustion products fromthe first stage are conducted through a porous matrix or bed ofrelatively inert refractory material. The gases are dispersed in thematrix or bed and are intimately mixed whereby the NO, and unburnedhydrocarbons react to form C and N Preferably, a catalyst is present inthe matrix to promote this reaction. The combustion gases including C0,,N and CO are'then passed through a second combustion stage where asource of oxygen, such as air, is introduced in any suitable manner suchas by injection into the gas stream. This oxidizes the residual unburnedhydrocarbons and carbonaceous material to CO in which case the exhaustgases are comprised mainly of harmless CO and N which may be dischargedto the atmosphere as non-pollutants. The dispersion bed or matrix andthesecond combustion stage are located in the combustion chamber so asto not impair the combustion process and to permit extraction of a largepercentage of the heat units available from the combustion gases foruseful purposes.

How the above-mentioned and other more specific objects of the inventionare achieved will appear in the detailed description of an illustrativeembodiment of the invention which will be set forth shortly hereinafterin reference to the drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical side elevation, partlyin section, of a boiler incorporating a preferred embodiment of theinvention;

FIG. 2 is a view taken along lines 33 in FIG. 1; and

FIG. 3 is a fragmentary view of a portion of the gas purifying meansillustrated in FIGS. 1 and 2;

FIG. 4 is a schematic representation of an alternate embodiment of theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS A use of the new combustion gaspurifier is exemplitied in the steam generating boiler 10 which is shownin FIG. 1. The boiler comprises an upper drum 11 which connects with alower feed water drum 12 by means of a plurality of water filled tubes13. These tubes together with the drums ll andlZ define a space in whichheat is exchanged between the hot combustion gases and the tubes anddrums. In the front lower region of the boiler there is a burner chamber14 lined with refractory material 15. A web 22 extends between each ofthe water tubes .to define a combustion chamber.

At the front end of the burner chamber 14 is an aperture 16 in which aburner (not shown) isinstalled. The burner may be any of theconventional types which are used for burning vaporized fluidhydrocarbon fuel or gaseousfuel in the presence of air.

Extending transversely across the combustion chamber is a gas purifyingassembly 17 consisting of a refractory holder 18 for containing gasdispersion bed 19. The bed 19 may be comprised of any suitablerefractory material which is substantially nonreactive with thecombustion gases within the expected gas temperature range. The bed 19should also be porous enough for combustion gases to be conductedtherethrough without substantial pressure drop or draft loss. Forexample, the bed 19 may comprise such refractory materials as siliconcarbide or alumina in pellet, chip, spheroidal, or other forms whichwill create a bed of relatively high porosity. In one nonexclusiveexample, the bed was formed of silicon carbide aggregate having anaverage size of about one quarter inch by one half inch loosely packedat random.

The refractory holder 18 may comprise a single member or in larger sizeunits, a plurality of members as shown in FIG. 1. Each of the holderunits 18 includes side and end walls 20 and 21 respectively, and abottom wall 23 for containing the bed material 19. The bottom wall 23 ofholder 18 is provided with suitable perforations 24 which permit thepassage of flue gas therethrough but which retain the bed material 19.While the perforations may take any convenient form, in the illustratedembodiment, they are shown to comprise a plurality of relatively short,closely spaced slots 24. The assembly 17 may be supported within theboiler 10 in any suitable manner such as by hollow water tubes 26 whichextend longitudinally at each side of the combustion chamber and whichmay be inclined upwardly from front to rear. A suitable insulatingspacer member 27 may be disposed between each of the water tubes 26 andthe holders 18.

The end walls 21 of holders 18 are provided with a plurality of arcuaterecesses 30 for supporting a plurality of elongate gas distributiontubes 31. This positions the tubes 31 in contact with the upper portionof the bed 19. The tubes 31 perform the function of distributing oxygenor an oxygen containing gas, such as air,-

along the upper portion of the bed 19. Toward this end, the tubes 31 maybe formed of any suitable heat resistant material which may be porous orperforated. For example, a porous refractory such as silicon carbide oralumina or a perforated metallic tube, such as stainless steel, may beemployed. The front end of each of tubes 31 are suitably connected to amanifold 32 which is in turn connected to a source of oxygen containinggas, such as air.

A plurality of spaced apart, refractory gas deflecting members 34 aresuitably positioned in general parallelism with air tubes 31. Themembers 34 may be fabricated from a single member or a plurality ofabutting members so that they will have substantially the same length asthe air tubes 31. As seen more particularly in FIG. 3, one member 34 ispositioned generally above and between each pair of adjacent air tubes31. In addition, the members 34 are formed in vertical cross sectionwith an arcuate surface 35 facing each of the adjacent air tubes 31 andspaced therefrom to form a gap 36 therebetween. A gas deflecting member37 is affixed at each side of the holder 18 and each has theconfiguration of approximately half of the members 34 to form a similargas directing gap 36 along each of said sides. The gas deflectingmembers 34 and 37 may be com posed of any suitable refractory material,such as silicon carbide. As will be described more fully hereinbelow,the gaps 36 between the tubes 31 and between said tubes and the gasdeflecting members 34 and 37 comprise a secondary combustion zone. Thespace below the tubes 31 comprises a primary combustion zone 38.

According to prior or conventional practice, an effort is made to mixsufficient air with the fuel to completely oxidize the combustiblecomponents of the fuel so as to produce, if possible, combustion gaseswhich have a high percentage of CO and a low percentage of unburnedhydrocarbons such as CO. Under such com bustion conditions, significantquantities of nitrogen oxide pollutants are also produced as a result ofnitrogen and oxygen from the combustion air combining at the hightemperatures normally present. It has been found that a substantiallygreater quantity of nitrogen oxides or NO, is produced in the absence ofunburned hydrocarbons such as CO at temperatures in excess ofapproximately 2500 F. Under conventional combustion conditions an excessof air is usually introduced in an effort to assure that at least thestoichiometric amount of oxygen will be available from the air tocompletely oxidize the carbon, hydrogen and other combustible elementsin the fuel at every point in the combustion zone. The excess oxygenpresent, particularly atomic oxygen, contributes to increased productionof NO, at elevated temperatures.

In accordance with the invention, combustion is con trolled in theprimary combustion zone 38 to insure the presence of unburnedhydrocarbons and/or the'lack of atomic oxygen. Because the reactionbetween carbon and oxygen is preferential to that'between oxygen andnitrogen, the presence of unburned hydrocarbons inhibits the formationof NO,. Toward this end the combustion in the primary combustion zone 38is controlled in any manner well known in the art, such as by adjustmentof the fuel-air ratio.

As seen in FIGS. 1 and 2, the stream of gaseous combustion productsidentified by reference numeral 42 passes upwardly through the slots 24and into the gas dispersion bed 19. There gases include a substantialamount of unburned hydrocarbons, such as CO and some nitrogen oxides. Inthe dispersion bed the gases are intimately mixed where the CO and NO,react to form N and CO In accordance with the invention, the gas exitingfrom the primary combustion zone 38 should include unburned hydrocarbonsand carbonaceous material as reflected by an excess of 400 ppm CO inorder to fully react with the N0 present. In one example, which has beensuccessfully employed, 2000 ppm CO were found to be satisfactory.

The refractory bed 19 may also include a catalyst to promote thereaction between CO and N0 Although the optimum concentration of thecatalyst in the bed 19 will vary in accordance with concentrations ofthe various gases in the combustion gas stream, in an illustrative casethe catalyst was in theform of metal strips of approximately l/64 inchby l/8 inch by several inches in length. The catalyst was dispersed in a3% inches thick bed of silicon carbide (SiC). Of course, the bedthickness and the amount of catalyst depends on combustion gas flowconditions and will differ from case to case. Suitable catalysts includeiron, nickel, chromium, copper and platinum as compounds or as alloysand mixtures thereof. This list of catalysts is not necessarilyexhaustive of catalysts which will promote the reaction between CO andN0 In the example given above, the catalytic metal strips were type 18-8stainless steel alloy which is effective as a catalyst because itcontains iron, nickel and chromium at least. The catalysts may also befused 'or dissolved in the silicon carbide or other refractory materialof which the bed 19 is comprised. Some grades of SiC may containimpurities which may serve as catalysts. The catalyst may also besprayed, metallized or otherwise deposited on the surface of therefractory material grains of the bed. The object in any case is tothoroughly disperse the combustion gases in the bed and to effectuateintimate contact between the gases and the catalytic agent if one isused.

The combustion gases evolving from the dispersion bed 19 includesignificant quantities of N CO and unburned hydrocarbons, such as CO.This residual unburned CO, which has not reacted with the NO, in the bed19, passes upwardly and into the secondary combustion zone 36 betweenthe air tubes 31 and the members 34. In addition, secondary combustionair, which evolves into the secondary combustion zone 36 from the airtubes 31 causes the complete oxidation of the unburned hydrocarbons toCO which is an exothermic reaction. The engagement between the air tubes31 and the bed 19 insures adequate mixing of the combustion air and thegases passing through the bed 19. Because nitrogen in air can beconverted to NO, at temperatures of approximately 2700 F or above, thetemperature in the secondary combustion zone 36 is maintained belowabout 2500 F. The temperature of zone 36 is dependent upon the radiationof heat away therefrom and the temperature and quantity of airdischarged from pipes 31. These factors are controlled to maintain thetemperature .in zone 36 below that wherein significant quantities of NO,are formed. The

radiation of heat away from the bed 19 is enhanced by its being exposedto the heat absorbing surfaces represented by the water tubes 13.

The means for reacting unburned hydrocarbons and carbonaceous materialwith NO, and for introducing secondary combustion air for oxidizing anyof the former that may remain to CO may be devised in a way that permitseliminating the bed of refractory material if desired. For instance,instead of the bed 19 of refractory aggregate materials an open celledmetallic sponge, a mat of gas transmissive metallic or refractoryfilamentary material, or monolithic porous refractory member can be usedto disperse the combustion gases and effect the reaction of unburnedhydrocarbons and- /or carbonaceous material and NO,,. A catalyst, ifdesired, can be incorporated in or deposited on the material of whichthe mat, sponge porous refractory is comprised. This could be in arefractory chamber in which the secondary air for oxidizing theunburned-hydrocarbons to CO and water is introduced remotely from themat or sponge so as to not overheat these elements.

As implied, the location, configurationand operating temperature of thenew combustion gas purifying device in a particular combustion apparatuswill depend on a number of variables including the primary and secondarycombustion rates, the quantity of hot gases to be handled, thetemperature of the primary combustion gases and the tolerable radiationfrom the device itself. In any case, it will be desirable to have thegas reside in the bed 19 and secondary combustion chamber 36 for a longenough period to obtain sufficient reactions between the unburnedhydrocarbons and the NO, in the bed and between CO and O and thesecondary combustion zone. The residence time of the gases in bed 19depends on the thickness, porosity and location of the bed 19, andreaction rate and temperature.

Generally, the amount of air injected in the secondary combustion stageneeded will depend on the unburned hydrocarbons discharged from theprimary combustion zone 38 and available for reaction in the secondarycombustion zone 36. In one embodiment, the secondary combustion airneeded amounted to about 10 percent of the stoichiometrically requiredair for complete combustion of the fuel. In other words, 90 percent ofthe total air for complete combustion was used in the primary stage andthe remaining 10 percent was injected in the secondary combustion stageto convert the residual CO to CO In reality, of course, more than thestoichiometrically required. amount of air is used since not all of theoxygen in the =air mixes thoroughly enough with the secondary combustiongases to become involved in the oxidation reactions.

The effectiveness of the invention in removing NO, from discharge gasesisillustrated with respect tothe test apparatus shown in FIG. 4. Herethe combustion device 50 includes a chamber 51 defined by a metallicshell 52 having a refractory lining 53. A suitable burner 54 is providedat the lower end of shell 51 and a discharge stack 55 adjacent its upperend. A gas dispersion bed 57 is disposed intermediate the ends ofchamber 51 and includes a refractory holder 58 for containing a siliconcarbide refractory aggregate 60. The lower end of holder 58 includes aplurality of suitable apertures 61 to permit the flow of gasestherethrough.

Secondary combustion air may be introduced through a porous refractorypipe 60 which is below the upper surface of the refractory bed 62.

In the experimental apparatus, number 2 fuel oil was burned in less thansufficient air to effect stoichiometric combustion. Measurements ofcarbon dioxide, carbon monoxide and oxygen were measured above and belowthe dispersion bed 57 and measurements of NO, were taken at points AKand O and temperature at points A-L as indicated in FIG. 2. Also,Bacharach numbers, which is a measurement of soot concentration, werealso taken above and below the dispersion bed 57. The following resultswere obtained;

BELOW ABOVE CO 15.0% of total 10.2% of total 0 0.2% of total 6.0% oftotal CO 2000 ppm None Detected Bacharach No. 9 2

POINT NOx ppm TEMP F A 96 2520 B 98 2600 C 89 2560 D 106 2560 E 105 2570F 107 2540 G 2570 H 86 2570 I 2500 J 88 25.10 K 84 2550 L 2300 M 2330 N2290 O 70 2040 those skilled in the art that the device and a comparableoperating mode may be adapted for use in various devices which fallwithin the generic class of fuel burning or reacting devices such asresidential and commercial furnaces and space heaters, hot waterheaters, incinerators, heating and power generating boilers,'othercombustion apparatus which burn liquid or gaseous hydrocarbon orcarbonaceous fuels. Thus, the foregoing description should be consideredillustrative of the manner in which the invention is used and the scopeof the invention should be governed by interpretation of the claimswhich follow.

We claim:

1. A combustion device adapted for reducing nitrogen oxides in itsexhaust gas, comprising:

a. an enclosure having fuel burning means and an outlet,

b. means within said enclosure for defining a first combustion zone,means for introducing fuel and oxygen into said first combustion zoneand for limiting the amount of oxygen to less than the amount requiredfor complete combustion to produce combustible products includingunburned hydrocarbons,

c. means within said enclosure for defining a second combustion zonecoupled to said first combustion zone for receiving combustion productstherefrom and including,gas pervious tube means disposed within saidsecond combustion zone for introducing oxygen at plural points into saidgaseous combustion products whereby to oxidize residual combustibleproducts including unburned hydrocarbons therein before discharge.

2. The invention set forth in claim l-and including means'formaintaining the temperature in said secondary combustion zone belowabout 2700F.

3. The invention set forth in claim 2 wherein said enclosure alsoincludes a gas dispersion means between said first combustion zone andsaid second combustion zone, said gas dispersion means being arranged sothat the combustion products emitted from said first combustion zonepass therethrough before being received in said second combustion zone.

4. The invention set forth in claim 3 wherein:

a. said gas dispersion means comprises a porous, re

fractory matrix.

5. The invention set forth in claim 4 wherein:

a. said matrix includes a catalyst for promoting the reaction ofnitrogen oxides with unburned hydrocarbon and carbonaceous materials.

6. The invention set forth in claim 4 wherein:

a. said refractory material matrix comprises silicon carbide fragmentsforming a porous bed.

7. The invention set forth in claim 3 wherein:

a. said gas pervious tube means comprise porous tubes of refractorymaterial.

8. The invention set forth in claim 3 wherein:

a. said gas dispersion means comprises a porous layer of catalyticmaterial.

9. The invention set forth in claim 3 wherein:

a. said dispersion means comprises a metallic element selected from thegroup-consisting of iron, nickel, chromium, copper and platinum andalloys thereof.

10. The invention set forth in claim 1 wherein said gas pervious tubemeans includes at least one hollow tube disposed in said second zone andhaving plural, spaced apart gas permeable means formed in said hollowtube forpermitting the flow of gas therethrough.

11. The invention set forth in claim 10 wherein said gas pervious tubemeans has a plurality of apertures formed therein.

12. The device set forth in claim 1 and including heat absorbing meanslocated in said enclosure downstream from said second combustion zonefor absorbing heat from said second combustion zone and for maintainingthe temperature in said second combustion zone below a predeterminedlevel.

13. The method of reducing nitrogen oxides in exhaust gases comprising:

providing a primary combustion zone,

incompletely oxidizing a fuel in said primary combustion zone to providegaseous combustion products including unburned hydrocarbons,

passing said gases through a second combustion zone,

introducing a source of oxygen into said secondary combustion zonewhereby to oxidize the residual unburned hydrocarbons in said streambefore discharge thereof, maintaining the temperature in said secondarycombustion zone at less than 2700F, said unburned hydrocarbon comprisescarbon monoxide having concentration in excess of 400 parts per million.10

gen oxides in its exhaust gas, comprising:

a. an enclosure having fuel burning means and an outlet,

b. a first combustion zone in said enclosure wherein fuel is burned withincomplete oxidation to produce combustible products including unburnedhydrocarbons and including gas dispersion means through which saidgaseous combustion products are conducted,

c. a second combustion zone including. porous silicon carbide tube meansfor introducing oxygen into said gaseous combustion products whereby tooxidize residual combustion products including unburned hydrocarbonstherein before discharge, and

3 gen oxides in its exhaust'gas, comprising:

for introducing fuel andless than the amount of ox ygen and nitrogencontaining gas required for complete combustion wherein fuel is burnedwith incomplete oxidation to produce combustible products includingunburned carbonaceous materials, means defining a second combustionzone,

means for conducting said combustible products to said second combustionzone,

said second combustion zone including plural tube means extending intosaid zone, said tube means being constructed and arranged to supply airto plural zones within said second combustion zone whereby to'oxidizeresidual combustible products including unburned carbonaceous materialstherein before discharge.

16. The combustion device set forth in claim 15 wherein said tube meansextends generally transversely to the direction of gas flow in saidsecondary zone whereby said gases from said primary zone are forced toflow around said tube means to promote mixing with said air.

17. The invention set forth in claim 16 and including a common manifoldconnected to at least one end of each of said tube means for deliveringair thereto.

18. The invention set forth in claim 17 and said tube means are formedof silicon carbide material.

19. The invention set forth in claim 18 wherein said tube means areporous. I ,0 20. The invention set forth in claim 19 wherein each ofsaid tube means has a plurality of apertures formed therein.

14. A combustion device adapted for reducing nitromeans defining a'firstcombustion zone, first means

2. The invention set forth in claim 1 and including means formaintaining the temperature in said secondary combustion zone belowabout 2700*F.
 3. The invention set forth in claim 2 wherein saidenclosure also includes a gas dispersion means between said firstcombustion zone and said second combustion zone, said gas dispersionmeans being arranged so that the combustion products emitted from saidfirst combustion zone pass therethrough before being received in saidsecond combustion zone.
 4. The invention set forth in claim 3 wherein:a. said gas dispersion means comprises a porous, refractory matrix. 5.The invention set forth in claim 4 wherein: a. said matrix includes acatalyst for promoting the reaction of nitrogen oxides with unburnedhydrocarbon and carbonaceous materials.
 6. The invention set forth inclaim 4 wherein: a. said refractory material matrix comprises siliconcarbide fragments forming a porous bed.
 7. The invention set forth inclaim 3 wherein: a. said gas pervious tube means comprise porous tubesof refractory material.
 8. The invention set forth in claim 3 wherein:a. said gas dispersion means comprises a porous layer of catalyticmaterial.
 9. The invention set forth in claim 3 wherein: a. saiddispersion means comprises a metallic element selected from the groupconsisting of iron, nickel, chromium, copper and platinum and alloysthereof.
 10. The invention set forth in claim 1 wherein said gaspervious tube means includes at least one hollow tube disposed in saidsecond zone and having plural, spaced apart gas permeable means formedin said hollow tube for permitting the flow of gas therethrough.
 11. Theinvention set forth in claim 10 wherein said gas pervious tube means hasa plurality of apertures formed therein.
 12. The device set forth inclaim 1 and including heat absorbing means located in said enclosuredownstream from said second combustion zone for absorbing heat from saidsecond combustion zone and for maintaining the temperature in saidsecond combustion zone below a predetermined level.
 13. The method ofreducing nitrogen oxides in exhaust gases comprising: providing aprimary combustion zone, incompletely oxidizing a fuel in said primarycombustion zone to provide gaseous combustion products includingunburned hydrocarbOns, passing said gases through a second combustionzone, introducing a source of oxygen into said secondary combustion zonewhereby to oxidize the residual unburned hydrocarbons in said streambefore discharge thereof, maintaining the temperature in said secondarycombustion zone at less than 2700*F, said unburned hydrocarbon comprisescarbon monoxide having concentration in excess of 400 parts per million.14. A combustion device adapted for reducing nitrogen oxides in itsexhaust gas, comprising: a. an enclosure having fuel burning means andan outlet, b. a first combustion zone in said enclosure wherein fuel isburned with incomplete oxidation to produce combustible productsincluding unburned hydrocarbons and including gas dispersion meansthrough which said gaseous combustion products are conducted, c. asecond combustion zone including porous silicon carbide tube means forintroducing oxygen into said gaseous combustion products whereby tooxidize residual combustion products including unburned hydrocarbonstherein before discharge, and d. means for maintaining the temperaturein said secondary combustion zone below about 2700*F.
 15. A combustiondevice adapted for reducing nitrogen oxides in its exhaust gas,comprising: means defining a first combustion zone, first means forintroducing fuel and less than the amount of oxygen and nitrogencontaining gas required for complete combustion wherein fuel is burnedwith incomplete oxidation to produce combustible products includingunburned carbonaceous materials, means defining a second combustionzone, means for conducting said combustible products to said secondcombustion zone, said second combustion zone including plural tube meansextending into said zone, said tube means being constructed and arrangedto supply air to plural zones within said second combustion zone wherebyto oxidize residual combustible products including unburned carbonaceousmaterials therein before discharge.
 16. The combustion device set forthin claim 15 wherein said tube means extends generally transversely tothe direction of gas flow in said secondary zone whereby said gases fromsaid primary zone are forced to flow around said tube means to promotemixing with said air.
 17. The invention set forth in claim 16 andincluding a common manifold connected to at least one end of each ofsaid tube means for delivering air thereto.
 18. The invention set forthin claim 17 and said tube means are formed of silicon carbide material.19. The invention set forth in claim 18 wherein said tube means areporous.
 20. The invention set forth in claim 19 wherein each of saidtube means has a plurality of apertures formed therein.